Concept
The traditional process for construction of a well first drills the hole section to depth followed by removal of the drill pipe, insertion of the casing and cementing of the casing in place. As noted previously, time and cost associated with tripping to perform this operation can be substantial, especially for deeper wells. The concept of drilling with casing allows for drilling and casing with the same tubular. There are currently two types of systems (retrievable & non-retrievable) commercially available for drilling with casing. There are additional service costs associated with each system. Both use standard available casing.
Originally conceived to save money by minimizing tripping costs, the commercial success of casing drilling has also been attributed to an improved ability to deal with lost circulation as compared to conventional drilling. One manifestation of the latter benefit is thought to arise from the so-called “plastering” effect in which the narrower annulus between the casing and borehole wall, as compared to conventional drill pipe, is theorized to produce an impermeable filter cake that mitigates lost circulation effects. This plastering effect is also thought to strengthen the borehole wall and reduce wellbore stability issues. Other benefits of casing drilling include a more reliable method for running casing all the way to the bottom of the hole and safer casing handling. A rapidly growing service is the use of casing drilling equipment to run casing on conventionally drilled holes.
Modeling of casing drilling costs on conventional geothermal wells less than 2,500 m indicate that casing drilling will probably not offer much in the way of cost savings compared to conventional practice14. The potential benefit of reducing lost circulation problems in geothermal drilling is speculative since there are radical differences between typical oil and gas and geothermal lost circulation zones. The reduction in trip time is not as significant where much of the drilling is shallow.
14 A.J. Mansure, “Advanced Drilling Concepts Final Report”, Internal Sandia Memo, 2008.
It is important to distinguish between cost savings when constructing the same well that can be drilled conventionally versus the impact of being able to construct a new well design not achievable by conventional technology. Casing drilling may offer the possibility for eliminating intermediate strings by controlling problems with wellbore stability in EGS wells greater than 3,000 m. This benefit could result in a significant cost savings.
The two commercial available casing drilling systems are described below.
Casing while Drilling (CwD) Systems
Retrievable System
Tesco provides drilling with casing systems that are retrievable for both vertical wells and directional wells. The standard components of the system for vertical wells are as follows: casing, casing profile nipple, casing shoe, casing stabilizers, drill lock assembly (DLA), under-reamer, and drill bit. The casing, casing profile nipple, casing stabilizers and the shoe make the casing string. The casing profile nipple is connected to the casing a certain distance above the casing shoe. This profile nipple enables the system to be retrievable. The casing stabilizers assist with deviation control. The bottom hole assembly (BHA) consists of an outer and inner string (Figure C-8). The inner BHA consists of the DLA, stabilizers, and spacer collar. The DLA is latched into the casing profile nipple. Thus, the DLA is completely enclosed in the casing string. The DLA is used to transmit torque from the rotating casing to the drill bit and to transfer axial loads (weight down) on the drill bit. Elastomers on the DLA external surface seal against the casing inside surface. This allow for pumping drilling fluid down the casing to the drill bit. A spacer collar is connected to the bottom end of the DLA. The spacer collar extends out below the casing shoe. Stabilizers above the spacer collar are used to stabilize the portion of the spacer collar that is located in the casing string. The external BHA consists of the under-reamer, spacer collar, stabilizers, and drill bit. The external BHA is attached to inner BHA spacer collar (the end that extends past the casing shoe) by the under- reamer. The under-reamer is specifically designed to be used in drilling with casing systems. It opens the pilot hole (created by the drill bit) by 50%. The annular space between the casing outer surface and the wall of the enlarged hole is cemented. Below the under-reamer is another collar with stabilizers. The drill bit used is typically a PDC type. Operation of the system first requires assembly of the inner and external BHAs by latching the internal section into the casing profile nipple. The casing is then lowered to bottom and rotated to commence drilling. Torque and axial loads are transferred from the casing to the drill bit by the DLA. While drilling, fluid is pumped down the casing, through the DLA, spacer collars, and under-reamer to the drill bit. The fluid is returned to surface in the annular space between the casing outer surface and the borehole wall. Once the casing point is reached, a wireline retrieving tool is deployed. The retrieving tool releases the DLA from the casing profile nipple. The BHA along with the DLA is then brought to surface. The drill bit used on the BHA is small enough to pass through the casing string. The casing is
then cemented. The next well interval is then ready to be drill in the same manner. Available sizes of casing drilling systems range from 4-1/2” to 13-3/8”.
C-8 Casing drilling bottom hole assembly
The directional system is very similar to the vertical drilling with casing system. A positive displacement motor (PDM) is placed below the DLA inside of the casing (above the under-reamer). Below the under-reamer is an MWD system with a rotary steerable system (RSS) above the drill bit. The directional system is considerably longer than the vertical system.
Non-Retrievable System
The system provided by Weatherford is a non-retrievable type system. A drillable drill bit is placed at the bottom of the casing string. No under-reamer is used in this system. The drill bit is able to open the hole sufficiently to allow for cementing of the casing after reaching depth. Once the casing is drilled to the required casing point, a ball is pumped down and lands on a ball seat in the drillable drill bit. Pressure is applied down the casing and the drill bit is shifted so that large flow ports allow for communication with the open hole. Cementing the casing then takes place. The next casing string then is run with a drill bit on bottom of it. The previous drill bit is drilled out and the wellbore drilling continues. With this system directional drilling is not possible.